Harddisk encryption with geli: key's block size

[Thomas Hobbes]

Hi,

I want to encrypt my mobile computer's data-partition with a
passphrase, 128 bit AES and HMAC/MD5. A lot of people use different
block sizes to generate keys with dd. There are examples with block
sizes of 64, 32k and 128k in geli's man-page, but I couldn't find out
why they were used. Spidering 'geli + "key bs"' discovered that there
are some more values used, i.e. 128, 512 and 1k. What is a reasonable
block size to use?

Bye
Thomas